Bistable display device having plural cavities containing rotatable bicolored particles within a liquid crystal carrier

ABSTRACT

A bistable display device comprises two cell walls enclosing a sheet having a plurality of cavities each containing a rotatable bicoloured sphere within a liquid crystal material carrier ( 15 ), and electrodes for applying an electrical field. The spheres and cavities are provided with a surface alignment which imposes a substantially unidirectional director alignment on the liquid crystal material in contact with them. The elastic distortion free energy of the liquid crystal material in each cavity is arranged to be zero when the preferred direction of the suspended spheres coincides with that of the surrounding cavity, e.g. black upwards. It is also zero when the sphere is rotated through 180° to present its opposite hemisphere which has a contrasting optical appearance, e.g. black downwards. Between these two states, there is an energetic barrier determined by the elastic constants of the liquid crystal which provides both a threshold for switching and improved bistability. During production of the sheet, alignment may be enhanced by application of an electric or magnetic field during curing or cooling of the components. The display may be addressed in a known multiplexed manner.

This invention concerns an electrically addressable display in whichparticles held in a matrix are rotated by an electric field,specifically a device having bistable switching.

A known device comprises an array of bi-coloured spheres suspended inliquid droplets in a plastic sheet. Opposite sides of the spheres carrydifferent electric charges: under an applied field the spheres can bemade to rotate, altering the apparent colour or reflectivity of thedevice. Magnetically actuated devices are also known. This device hasbeen described as a “Gyricon” device; details are in U.S. Pat. Nos.4,126,854, 4,143,103, 5,075,186, 5,262,098, 5,344,594, and 5,389,945 andare incorporated herein by reference.

Gyricon devices show little or no true threshold or bistabilitybehaviour. The image once written is stable provided no field isapplied, but the writing characteristics of the device make multiplexdrive difficult. Complex devices must incorporate an active matrix drivecircuit. This limits the utility of the technology for applications suchas information display and electronic paper.

The problem of poor bistability and no true threshold are solvedaccording to this invention by suspending the spheres in a liquidcrystal material and surface treating the spheres and cavities toprovide liquid crystal director alignment.

According to this invention a bistable display device comprises two cellwalls enclosing a sheet having a plurality of cavities each containing arotatable bicoloured particle within a liquid carrier, and electrodesfor applying an electrical field,

Characterised In That

the liquid carrier is a liquid crystal material and

the spheres together with the cavities are provided with a surfacealignment which imposes a substantially unidirectional directoralignment on the liquid crystal material in contact with them,

whereby the particles have two switchable bistable states with an energybarrier between the two states.

The liquid crystal material is preferably a nematic material of eitherpositive or negative dielectric anisotropy.

The spherical particles have a diameter of less than 10 μm, e.g. lessthan about 45 μm, typically about 10 μm.

The thickness of the sheet may be about 50 to 1000 μm.

The cell walls made be rigid glass or plastic, or thin flexible plasticmaterial.

The invention will now be described, by way of example only, withreference to the accompanying drawings of which:

FIG. 1 is a plan view of a matrix multiplexed addressed display havingspherical particles within a plastic sheet;

FIG. 2 is a cross section of the display of FIG. 1;

FIG. 3 is an enlarged cross section.

The display in FIGS. 1, 2 comprises a display cell 1 formed by a sheet 2of plastic material contained between glass walls 3, 4 and (optionally)a spacer ring 5. The walls 3, 4 are typically 50-1000 μm apart, e.g.about 500 μm. Strip like row electrodes 6 e.g. of SnO₂ or ITO (indiumtin oxide) are formed on one wall 4 and similar column electrodes 7 areformed on the other wall 3. With m-row and n-column electrodes thisforms an m×n matrix of addressable elements or pixels. Each pixel isformed by the intersection of a row and column electrode. Either or bothelectrodes 6, 7 are optically transparent.

A row driver 8 supplies voltage to each row electrode 6. Similarly acolumn driver 9 supplies voltages to each column electrode 7. Control ofapplied voltages is from a control logic 10 which receives power from avoltage source 11 and timing from a clock 12.

The plastic sheet 2 is optically transparent and carries numerousspherical cavities 13 filled by spherically shaped particles 14 floatingin a liquid crystal material 15 such as the nematic liquid crystalmixture E7, which is commercially available and which comprises amixture of cyanobiphenyls and cyanoterphenyls of positive dielectricanisotropy. One side of each particle 14 is coloured differently to thatof the other side; for example black on one side and white on the other.Other colour combinations may be used.

The particles 14 may be smooth or rough surfaced on both sides, orsmooth on one side and rough on the other side. One side of theparticles may be highly absorbing whilst the other is highly reflecting.Particles in a dielectric liquid acquire an electric charge related tothe Zeta potential of their surface coating. Corresponding to theoptical anisotropy of the particles, different surfaces presentdifferent Zeta potentials, with the result that the particles have anelectrical anisotropy in addition to the optical anisotropy.

Application of an electrical field results in rotation of the sphericalparticles and a change in optical appearance. For example a positivefield may produce black upwards (left hand side of FIG. 3) and anegative field produce white upwards (right hand side of FIG. 3). Thusby application of voltages to each row and column electrode in amultiplex addressing manner, each pixel in the display may be addressedto form a desired display pattern. Multiplexed addressing may beachieved by line at a time application of a voltage which exceeds thethreshold voltage, to selected pixels of the device. Means to achievethis addressing, and to maintain the non-select error voltage on thedevice below the threshold voltage are well known to those skilled inthe art.

The spheres may be produced as described in the prior art. For exampleby application of two differently pigmented plastic liquids to oppositesides of a spinning disk. Centrifugal force causes the two liquids toflow towards the periphery of the disk where they combine at the edge toform bichromal ligaments that eventually break up as bichromal spheres,i.e. optically anisotropic spheres. Alternatively, microspheres may bepre-formed and subsequently coated with a coloured material e.g. bydirectional vacuum deposition. The deposited material may simultaneouslyalter both the colour and the zeta potential of one hemisphere of eachsphere. A suitable sphere may be obtained by evaporation of copperphthalocyanine pigment onto one hemisphere of polymer spheres composedof poly(methylmethacrylate) containing 15% of titanium dioxide pigment.

Prior to incorporation or after incorporation in the matrix sheet 2, thespheres are surface treated to give alignment to liquid crystalmolecules. Such alignment may be achieved by forming them from analigned liquid crystalline monomer mixture by thermal or UVpolymerisation or crosslinking. An aligned liquid crystal polymer mayalso be provided as a surface coating on isotropic spheres by similarmeans. A suitable alignment may be achieved by coating the spheres witha ca. 100 nm layer of 4-cyano-4′-(3-methacryloylpropyloxy) biphenyl,placing them in a magnetic field of 5T aligned with the axis of opticalanisotropy, and exposing the spheres to actinic light. A directionalsurface may also be achieved on spheres by forming or cooling them insuspension in an aligned liquid crystal, the spheres themselves beingformed of an isotropic polymer. Alternatively known surface treatmentsmay be applied such as deposited and patterned polymers and surfactants.Alternatively surface texturing may be applied to provide liquid crystalalignment.

The sheet 2 incorporating the spheres 14 may be formed by mixing thespheres with an uncured liquid elastomer and curing by thermal or UVpolymerisation to give an alignment to liquid crystal material withinthe cavities 13. When cured, the sheet 2 is placed in a dielectricliquid plasticiser which is also a liquid crystalline material such asthe E7 liquid crystal referred to above. After immersion for some time,the plasticiser becomes absorbed into the sheet, thereby swelling thesheet and filling the space between spheres 14 and cavity walls 13. Theuncured liquid elastomer may be a commercially available siliconeprepolymer such as Sylgard 184 or an epoxy prepolymer such as abisphenol-A/epichlorhydrin condensate. Alternatively, the sheet 2incorporating the spheres 14 may be formed by mixing the spheres with asoluble polymer dissolved in a solvent in which the spheres aresubstantially insoluble, and removal of the solvent. The sheet is thenswollen with a liquid crystalline fluid as above.

The curing or solvent evaporation steps, as well as the swelling of thelayer with a liquid crystalline fluid may be performed in a field, e.g.,an electric field or a magnetic field which serve to impose a preferredalignment direction on a liquid crystal phase. Typically said alignmentwill also be imprinted on adjacent solid surfaces in the absence of astrongly anchoring alignment layer.

Optionally the polymer layer may be composed of a liquid crystallinepolymer formed into a layer by curing or by solvent evaporation at sucha temperature that the polymer is in an isotropic state. Subsequentcooling, optionally in an applied field, will impose directional orderon the polymer and on solid surfaces in contact with it. Optionally sucha liquid crystalline polymer may be formed on a substrate or betweensubstrates treated to impose an alignment, e.g., a homeotropic alignmenton the phase. The swelling of the layer may also be performed with aliquid crystalline fluid at such a temperature that it is in anisotropic state, and forms the liquid crystal state on cooling.Optionally a field may be imposed during this cooling stage. Fields, ifapplied, may be generated by known means including permanent orelectromagnets, secondary electrodes connected to a high voltage, or bycorona discharge.

Suitable liquid crystalline polymers include sidechain polymers based onsiloxane, acrylate, methacrylate, poly(oxirane) and poly(oxetane) chainssubstituted with cyanobiphenyl, ester, biphenyl, azoxy,phenylcyclohexane and biphenylcyclohexane groups. Optionally a spacerunit is included between the liquid crystal sidegroup and the mainpolymer chain. Main chain liquid crystal polymers may also be used.

The cavity surface alignment together with that on each sphere imposes asubstantially unidirectional director alignment on a liquid crystal incontact with them. The elastic distortion free energy of the liquidcrystal material in each cavity is arranged to be minimised when thepreferred direction of the suspended sphere coincides with that of thesurrounding cavity, e.g. black upwards. It is also minimised when thesphere is rotated through 180° to present its opposite hemisphere whichhas a contrasting optical appearance, e.g. black downwards. Betweenthese two states, there is an energetic barrier determined by theelastic constants of the liquid crystal which provides both a thresholdfor switching and improved bistability.

It is not necessary for a strictly unidirectional and uniform alignmentto be achieved at each surface. For example, alignment of the liquidcrystal to form defect pairs at defined points on the surface of thespheres, known as a Boojum configuration, and corresponding alignment onthe interior of the droplets, will also suffice.

1. A bistable display device comprising two cell walls enclosing a sheethaving a plurality of cavities each containing a rotatable bicolouredparticle within a liquid carrier and electrodes for applying anelectrical field, wherein the liquid carrier is a liquid crystalmaterial and the particles together with the cavities are provided witha surface alignment which imposes a substantially unidirectionaldirector alignment on the liquid crystal material in contact with them,whereby the particles have two switchable bistable states with an energybarrier between the two states.
 2. The device of claim 1 wherein theparticles are of spherical form.
 3. The device of claim 1 wherein theparticles are of ellipsoidal form.
 4. The device of claim 1 wherein theliquid crystal material is a nematic material of either positive ornegative dielectric anisotropy.
 5. The device of claim 1 wherein thespherical particles have a diameter of less than 100 μm, e.g. less thanabout 45 μm, typically about 10 μm.
 6. The device of claim 1 wherein thespherical particles have a diameter of less than about 45 μm.
 7. Thedevice of claim 1 wherein the thickness of the sheet is 50 to 1000 μm.8. The device of claim 1 wherein the cell walls are made of rigid glassor plastic, or thin flexible plastic material.
 9. The device of claim 1wherein the liquid crystal material is a material selected from the listcontaining nematic liquid crystal, smectic A liquid crystal, smectic Cliquid crystal, chiral nematic liquid crystal.
 10. The device of claim 1wherein the sheet is formed of a liquid crystal sidechain polymer.
 11. Amethod of manufacturing a bistable display device, the method comprisingthe steps of: providing a sheet having a plurality of cavities, eachcontaining a rotatable bicoloured particle within a liquid carrier;enclosing the sheet between two cell walls; and providing electrodes forapplying an electrical field; wherein the liquid carrier is a liquidcrystal material and the particles and the cavities are provided with asurface alignment which imposes a substantially unidirectional directoralignment on the liquid crystal material in contact with them, wherebythe particles have two switchable bistable states with an energy barrierbetween the two states.
 12. A method according to claim 11 in which therotatable bicoloured particles are surface treated, to give the surfacealignment to the liquid crystal materials, before incorporation of theparticles in the sheet.